1. Field of the Invention
This invention relates to the field of image data recording and transmission. More particularly, this invention relates image data recording and transmission in which image data is stored prior to transmission over a bandwidth limited transmission channel.
2. Description of the Prior Art
Known systems used in broadcast situations store high quality video image data on magnetic recording tape. The video image data is then read from the magnetic recording tape and transformed into a transmission signal format of reduced bandwidth.
A significant limiting performance parameter of most image data storage devices is the amount of data or number of images that can be stored. One way of decreasing the amount of space needed to store an image is to compress the image data with a technique such as that proposed by the Joint Photographic Experts Group and currently under review by the International Standards Organisation. It is an object of the invention to increase the number of images that can be stored.
Viewed from one aspect the invention provides an image data recording and transmission apparatus having:
(i) a decorrelator for producing frequency separated component data from input image data;
(ii) a recorder for storing frequency separated component data produced by said decorrelator;
(iii) an interpolator for producing reconstructed data from frequency component data stored by said recorder; and
(iv) a transmission signal generator for producing a bandwidth limited transmission signal from reconstructed data produced by said interpolator,
wherein said recorder stores substantially only that portion of frequency component data produced by said decorrelator corresponding to frequencies transmittable with said transmission signal.
The invention recognises that in a system in which the image data will be transmitted with a bandwidth limited signal there is no point in preserving information in the image data that cannot be represented by the transmission signal. Discarding such information does not degrade the quality of the image recoverable from the transmission signal, and yet allows an improved degree of data compression to be achieved.
In addition to recognising the above, the invention provides a particularly simple and efficient way of removing the excess data since for other data compression purposes the input image data may well have already undergone frequency separation. In this case, the overhead required to carry out the frequency separation is already present in the system and a relatively small modification in accordance with the invention allows the additional advantage of the invention to be realised.
In order to match the known NTSC transmission signal format said recorder stores substantially only that portion of frequency component data produced by said decorrelator corresponding to frequencies less than substantially 4.5 MHz.
Similarly, in order to match the known PAL transmission signal format said recorder stores substantially only that portion of frequency component data produced by said decorrelator corresponding to frequencies less than substantially 5.5 MHz.
Preferred embodiments of the invention enhance the data compression achieved with an entropy encoder for encoding frequency separated data prior to storage by said recorder and an entropy decoder for decoding frequency separated data read from said recorder.
Further improvements in data compression are achieved with a quantiser for quantising frequency separated data prior to storage by said recorder and a dequantiser for dequantising frequency separated data read from said recorder.
It will be appreciated that the frequency separation could be carried out by discrete cosine transformation of block of the image. However, improved performance is achieved when said decorrelator produces sub band component data representing different portions of the two-dimensional spatial frequency domain.
Since the decorrelation works on the basis of a two fold frequency division at each stage, an upper limit on the frequency of data extracted cannot be easily set if good frequency resolution is to be preserved at lower frequencies, e.g. if the signal undergoes a three stage frequency separation: to meet the required frequency resolution, it may be that the two uppermost bands are above the frequency that can be carried by the transmission channel. Accordingly, the output from the decorrelator may include high frequency information not required. One simple way of dealing with this is to configure the system such that said quantiser quantises sub band component data not transmittable with said transmission signal to a single value. Quantising to a single value, such as zero, effectively removes the unwanted information.
Preferred embodiments of the invention carry out the subsequent required filtering to remove the unwanted high frequency data utilising a data sequencer for sequencing sub band component data to separate sub band component data transmittable with said transmission signal from sub band component data not transmittable with said transmission signal.
In some embodiments said data sequencer removes sub band component data not transmittable with said transmission signal. Alternatively, said data sequencer sequences said sub band component data into an order in which sub band component data transmittable with said transmission signal is fed to said entropy encoder before sub band component data not transmittable with said transmission signal. Sequencing the data in this way allows an increase in the degree of compression achieved through entropy encoding by increasing the probability of long zero value run lengths and the occurrence of end of block codes.
Viewed from another aspect the invention provides an image data recording and transmission method comprising the steps of:
(i) decorrelating input image data to produce frequency separated component data;
(ii) storing frequency separated component data;
(iii) producing reconstructed data from stored frequency component data; and
(iv) producing a bandwidth limited transmission signal from reconstructed data,
wherein substantially only that portion of any frequency component data produced corresponding to frequencies transmittable with said transmission signal is stored.
The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings.